FIELD OF THE INVENTION
This invention relates in general to single crystals and in particular to a new and useful single crystal with at least one device for anchoring heating elements of an indirect resistance heater, especially thermionic emission cathodes for optoelectronic applications, preferably consisting of a boride or mixed boride of an element from the rare earth series and to a process for anchoring a heating element of an indirect resistance heater in a single crystal and the use of such single crystals.
Single crystals are heated by the principle of indirect resistance heating for very diverse areas of application. The permanent and maintenance-free anchoring of the heating element in the crystal body usually encounters difficulties. For optoelectronic applications, for example, single crystals of lanthanum hexaboride (LaB.sub.6) or other borides are used as emission cathodes and are ordinarily heated by an indirect resistance heater, which represents a power lead at the same time, to an operating temperature between 1500.degree. C. and 1600.degree. C. In this range, the pitting of the crystal substances by vaporization becomes less important compared to the oxidation with residual gas containing O.sub.2 and the subsequent vaporization of the boroxide and lanthanoxide. Accordingly, even at the high vacuum of 1.times.10.sup.-5 pascal, the rate of attrition by oxidation is of the same order of magnitude as the rate of attrition by vaporization of LaB.sub.6.
The result of this oxidation vaporization is that the hexaboride cathodes in all of the mounts that have been proposed up to this time more or less rapidly become loose in the course of time because of splitting if the vacuum is not better than 10.sup.-5 pascal.
As soon as a gap forms between the cathode and its mount, the positions of the cathode and its cathode spot change. The best transfer that now occurs partly by radiation and partly be thermal conduction also becomes unstable. If the cathode is included in the heater current path, the contact resistance also changes. Both of these lead to temperature variations with corresponding fluctuations of the emission.
Splitting can also not be avoided by a mount in which the LaB.sub.6 cathode is clamped between two pyrolytic graphite jaws and the power feed and heating are accomplished by these jaws under spring pressure. It leads to a steady, frequently also sudden change of the contact resistance. This design is also very costly and requires substantially more space than a hairpin cathode.
Another method is disclosed by German Patent Application Disclosure 32 03 917 A1. According to it, the splitting between an LaB.sub.6 cathode and its mount is overcome by joining the mount designed as a U-shaped bow and made of high-melting metal to the precisely matched LaB.sub.6 single crystal that is used as the cathode, by sintering. To prevent a reaction between the cathode and the metal bow, a thin layer that consists of colloidal carbon and a reaction barrier material is introduced as a paste between the surfaces that are to be joined to one another. Since such an interlayer is brittle after the sintering, it cannot be exposed to any mechanical stress in operation. This requires long, flexible power leads. It was also stated here that because of the different coefficients of expansion of the mount and the cathode, cracks nevertheless form in the course of time that lead to a gradual impairment of the heat transfer from the heating element to the cathode, and ultimately to the loosening of the cathode.
Similar difficulties occur in other applications of heated single crystals.